soils

soils It is possible to define soil in many ways. A useful, well-established, and all-embracing definition proposed by J. S. Joffe in 1945 is that soil is a natural body consisting of layers or horizons of mineral and/or organic constituents of variable thickness which differ from the parent material in their morphology, physical, chemical, and mineralogical properties and their biological characteristics. A soil can be described in terms of its profile, which is the vertical arrangement of the soil horizons down to parent material. The fact that soils can be grouped into broad classes with similar properties suggests that all soils are influenced by a universal set of factors. These are known as state factors.

Factors of soil formation

Soils develop as a result of the interplay of five factors of soil formation: parent material, climate, organisms, topography, and time. These were incorporated by Hans Jenny in 1941, in a state factor equation:S or s = f (c, o, r, p, t)

where S denotes the soil, s is any soil property, c is the climate factor, o is the organism or biotic factor, r is the relief or topography factor, p is the parent material, and t is the time factor. Jenny defined these factors in the following terms. The climate factor represents the regional climate, with precipitation and temperature being considered as separate functions. The organism or biotic factor is essentially vegetation and is the summation of the plant matter reaching the soil. The topographic factor includes the shape and slope of the landscape, the aspect of the slope and the height of the water-table, the latter usually being related to topography. Parent material includes both weathered and unweathered material from which the soil formed. This includes material both in situ and transported and may also include a pre-existing soil. Time is the time elapsed since the deposition of material, the exposure of material at the surface, or the formation of the slope.

A number of attempts have been made to show that some factors are more important than others. In the early days of soil science, rock type was thought to be the most important factor. Early Russian workers stressed the importance of climate, and many soil classification schemes and world soil maps were based on climate characteristics. Each factor is, however, essential; none can be considered generally to be more important than any other, although locally one factor may exert a strong influence (see soils and topography). These factors define the ‘state’ of the soil system and provide the framework within which soil processes operate.

Soil characteristics

Undisturbed soil is a mixture of organic and inorganic solid particles and interconnected voids containing varying amounts of soil, water and gases. The appearance of a soil is influenced by a large number of characteristics, of which only the more significant ones can be examined here. Colour is often the most obvious characteristic and may indicate some of the processes that are, or were, operating within the soil. Dark colours near the surface usually indicate an accumulation of organic matter. Yellow-brown to red colours generally indicate the presence of iron (ferric) oxides, and greyish colours are usually due to ferrous iron compounds formed under reducing (gleying) conditions. White or light grey colours characterize a horizon from which leaching has removed oxides or hydroxides of aluminium or iron.

The textures of soils reflect the proportion of sand, silt, and clay sizes within that portion of an inorganic soil fraction that is less than 2 mm. The relative combinations of sand, silt, and clay are the formal basis of the various soil textural classes (Fig. 1). Texture will affect processes operating within the soil, and will affect chemical exchange because surface area per unit volume increases greatly as particle size decreases. Organic matter, which is usually concentrated near the surface, ranges from undecomposed plant and animal tissue to humus—a relatively resistant mixture of brown and dark-brown amorphous and colloidal substances modified from the original organic matter by various soil organisms. Carbon usually makes up over one-half of organic matter and the carbon–nitrogen ratio is a good indication of the amount of decomposition of the original organic material. The ratio is high (greater than 20) in plant tissue and low (less than 10) in humus. Soil organic matter increases the water-holding capacity of soils and the cation-exchange capacity. Carbon dioxide is released during the formation of humus, which aids the formation of carbonic acid, increases soil acidity, and enhances weathering.

Soil structure reflects the way in which individual particles are aggregated together. The individual aggregates, called peds, are classified into a number of basic types (Fig. 2). Texture can be an important control here; clay content is important in the creation of blocky, prismatic, and columnar structures. The movement of water through the soil and surface erosion are very much affected by structure. Some aggregates are unstable in water, breaking up and allowing clay to be removed by percolating water; some of this clay may be redeposited as clay skins on ped surfaces lower down the profile.

Water is an important constituent of soils. It is held in the soil by adhesive forces between water molecules and organic and inorganic particles, and by cohesive forces between adjacent water molecules. Moisture content is usually expressed as a percentage of the oven-dry soil weight, but it can also be related to its availability to plants. When the forces holding water films on soil-particle surfaces equal the forces of downward gravitational pull, the soil is said to be at field capacity. Water is removed easily from a soil by evaporation and transpiration through the vegetation. As more water is removed from the soil, water films become thinner and are held by ever-increasing forces of attraction. Eventually, the water is held so strongly that roots cannot extract it. The water content at which this occurs is called the permanent wilting point. Water retention is largely related to the organic matter and clay contents, whereas water movement is influenced by bulk density, porosity, and permeability; bulk density is the weight of soil per unit volume; porosity is the percentage of voids to the total volume of soil; and permeability is a measure of the ease with which water can pass through the soil.

Soil acidity, or pH, is an important characteristic and normally varies from 5 to 9. A value of 7 indicates neutral conditions, below 7 indicates acid conditions, and above 7 indicates alkaline conditions. Soil acidity is largely a function of organic matter and the type and amount of cations. Large amounts of organic matter tend to produce acid conditions unless counterbalanced by basic cations. Hydrogen and aluminium ions are largely responsible for soil acidity; aluminium is released by the weathering processes of hydrolysis and solution, and the production of free hydrogen ions increases the acidity. pH tends to decrease as rainfall increases because leaching of basic cations is then increased.

Soil horizons

The soil characteristics discussed above are not distributed randomly in the soil profile but are generally organized to produce a definite vertical and lateral structure to the system. Horizons are layers differentiated vertically in the soil body that differ in their physical, chemical, and biological attributes. The distinctiveness of soil type is usually related to the properties of its horizons, and horizon designations are an element in the definition of soil units and in the description of representative profiles.

A consistent set of master horizons, designated by capital letters, H, O, A, E, B, C, and R, with distinctive characteristics can be described (Fig. 3). The upper part of a soil is usually an organic horizon (O), formed or forming from accumulations of organic matter deposited at the surface. It may be subdivided into fresh litter (L) deposited during the most recent phases of litter fall and the partly decomposing litter (F) remaining from earlier periods of litter fall; this is also known as the fermentation layer. In the next layer (H) the organic matter is well decomposed and original plant structures cannot be discerned.

The A horizon is the first dominantly mineral horizon, in which humified organic matter is mixed with the mineral fraction. The organic matter is either distributed as fine particles or occurs as coatings on the mineral particles. A horizons are usually quite dark in colour because of the organic content. An E horizon, which usually underlies an O, H, or A horizon, is an ‘eluvial’ horizon (see soil development), having lost compounds of iron and aluminium (sesquioxides) and silicate clay by leaching and translocation. It contains a lower organic content than the A horizon and is usually paler in colour. The B horizon is commonly a mineral horizon characterized by weathering of the original parent material in situ. It may possess an ‘illuvial’ (see soil development) concentration of silicate clay, iron, aluminium, or humus, alone or in combinations. It is also characterized by the release of sesquioxides by weathering and is the zone where large-scale granular, blocky, or prismatic structures are formed. B horizons are the most variable of soil horizons. The C horizon is the parent material from which the soil has been derived. It lacks the properties of A and B horizons but includes weathering, as indicated by mineral oxidation, accumulation of silica, carbonates, or more soluble salts, and is often gleyed. The R horizon is usually hard, unweathered bedrock.

It is sometimes necessary to qualify the master horizon description with a suffix, or suffixes, to indicate additional properties of a horizon. Thus Ap indicates that the A horizon has been mixed by ploughing, Ag signifies gleying, and Ah denotes an accumulation of organic matter. Suffixes are used extensively in the B horizon to represent alteration by weathering in situ (Bw), illuvial concentration of clay (Bt) and sesquioxides (Bs), or a cemented layer (Bm). In this way a detailed description of a soil profile is also an interpretation of the processes that have shaped the profile. The nature and arrangement of horizons also form the basis for most soil classifications.

Soil classification

It is necessary to stress that there is no international agreement about the classification of soil types and there are at least ten different systems in use in various countries. Early Russian workers established that there was a close relationship between soils and vegetation and between soils and climate. This led the famous Russian soil scientist Dokuchaev to propose a classification based on broad vegetation zones, such as boreal, taiga, forest–steppe, steppe, etc. Such soils were called zonal soils, but it was soon apparent that soil types were not completely uniform over these broad areas. Because of this, soils that were formed as a result of the influence of a specific local factor, such as parent material or topography, were called intrazonal soils and young or poorly developed soils, such as those developed on river alluvium, were termed azonal soils. The division into zonal, intrazonal, and azonal soils was the basis for the soil classification used in the USA until 1960. It contains soil types, such as podzols, chernozems, brown earths, chestnut soils, etc., familiar to many people. Because it was based on zonal concepts this classification was very attractive to soil mappers, geographers, and ecologists. But such a scheme has severe limitations. Soils, such as podzols, which are zonal in some parts of the world might be intrazonal in other areas. Also, the system was based on environmental factors rather than on the essential characteristics of the soils. This type of classification has now been superseded by the United States Soil Conservation Service's Soil Taxonomy scheme and by an FAO–UNESCO classification.

The Soil Taxonomy scheme has six levels of categorization: order, suborder, great group, subgroup, family, and series. There are ten orders, differentiated by the presence or absence of certain diagnostic horizons or features that demonstrate which soil-forming processes have been dominant. It is a complicated system which often requires detailed measurement of the soil profile and quite sophisticated laboratory determinations, but it is a flexible system which is being adapted continuously. The FAO–UNESCO scheme was developed for use as the basis for the production of a world soil map at a scale of 1 : 50 000 00, which was completed in 1981. In this scheme, soils were divided into 26 major groups at the first level of the classification and into 106 soil units at the second level. This classification is also based on observable or measurable attributes of soils. The scheme was revised in 1988, and the number of major groups was then increased to 28 and the number of soil units to 153. These changes demonstrate the fluid nature of soil classifications and the great variability of soil types, which reflects the interaction of the soil-forming factors discussed above.

John Gerrard

Bibliography

FAO–UNESCO (1974) Soil map of the world, Vol. 1. FAO–UNESCO, Paris.
FitzPatrick, E. A. (1980) Soils: their formation, classification and distribution. Longman, Harlow.
USDA (1975) Soil taxonomy. Agricultural Handbook No. 18. USDA, Washington, DC.